Solenoid actuators, such as solenoid valves, are common in many applications. These applications include automobiles, including but not limited to, the power train. In use, it is often advantageous to know whether the actuator is open or closed, or more precisely whether the actuator position (i.e. open or closed) has changed. Knowledge of the actuator position allows for an error check to make sure that an actuator that is supposed to open is actually open and a actuator that is supposed to be closed is actually closed.
A solenoid actuator includes a solenoid wrapped around a solenoid core, which is attached to a valve, gear, or other device to be moved. When a current is applied to the solenoid, the magnetic field from the solenoid produces a force on the solenoid core, moving the actuator open or closed. When the ferrous core (typically iron or steel) begins to move, the inductive properties of the coil change, affecting the back EMF and the resultant current through the inductor. By monitoring the change in inductance of the solenoid, the mechanical motion can be detected for diagnostic purposes. Typically the current is monitored for this change in inductance, however, other methods have also been employed. One method of measuring the current is to measured the voltage differential across a sense resistor connected in series with the solenoid.
The current through the solenoid changes as the solenoid is energized and the core moves. As the current is applied to the solenoid, the current profile changes based on the inductive properties of the solenoid coil, and proceeds through three zones—a base state, a changing state, and a changed state. The position of the actuator can be determined in response to the slope of the current profile, as illustrated in FIGS. 1A and 1B. When the slope of the current profile changes from a positive slope (i.e., the first zone or base state) to a negative slope (i.e. in the second zone or the changing zone), the actuator is changing its position, and when the current profile resumes a positive slope in the third zone, the actuator has completed a change.
Present systems for detecting solenoid actuator motion rely on a local maximum and local minimum found in the measured current profile for a solenoid actuator. As seen in the plot of measured current with time 110 in FIG. 1A, the measured current increases in Zone 120 when the power to the solenoid is switched ON and reaches a local maximum. The measured current peaks and declines to a local minimum in Zone 130 as the core begins to move. The measured current increases from the local minimum in Zone 140 and asymptotically approaches a steady state value as the valve is moving. The plot of measured current with time 110 in FIG. 1B shows the case in which the core fails to move when the power to the solenoid is switched ON. The inductance of the core stays constant and the measured current increases smoothly, without a maximum or minimum. One example of verifying solenoid operation is U.S. Pat. No. 5,808,471 to Rooke. Rooke relies on the local maximum, the local minimum and the increase after the minimum to detect that the core is moving.
Unfortunately, not all solenoid valves exhibit the current profile of FIG. 1A. Only solenoid valves with relatively high inductance exhibit the local maximum and minimum. High inductance solenoid valves are not desirable in all applications, so the present systems for detecting solenoid actuator motion cannot be used for all applications. In contrast to the current profile exhibited in high inductance actuators, the current profile exhibited in lower inductance actuators is more similar to that illustrated in FIG. 2.
Another problem for the present systems is solenoid valves controlled by Pulse Width Modulation (PWM) circuitry. PWM controlled valves cycle on and off too quickly to accurately measure a local maximum and minimum. Peaks are not caused by inductance but by the cycling current inherent in PWM circuits. present systems perform best with smooth waveforms, but PWM controlled valves include high frequency components as shown in FIG. 1C and in a more detailed view in FIG. 1D.
Therefore, it would be desirable to provide a method for determining changes in actuator position that would overcome the aforementioned and other disadvantages.